Light Emitting Diodes (LEDs) are desirable for use in lighting fixtures due to the efficiency and reliability of LEDs. LEDs used for interior lighting are typically high output devices that emit light that is a “pure” white (or nearly white) color. This color and output level work well for situations where bright lighting is desired. Some modern LED interior lights have a dimming feature for when lower light levels are desired. However, prior LED dimming systems do not closely follow the dimming color curve of an incandescent light, particularly at low and very low dimming levels. Also, the color of an LED does not change appreciably when the LED is dimmed, as does an incandescent light.
Unlike LEDs, traditional incandescent bulbs change color as they dim. Normally, the filament in an incandescent bulb emits a light with a color temperature of about 3000 Kelvin (K) at full brightness, which is considered a relatively “cool” white color. As the incandescent light is dimmed and the current is decreased, the filament emits a light that shifts away from “cool” white toward a “warm” color, such as red/amber color output (e.g., a lower color temperature).
The color or appearance of a light source can be defined as a color temperature and is measured in degrees Kelvin (K). For example, a fluorescent light may have a very “cold” color temperature of 4000K (which may appear bluish), whereas a standard incandescent light bulb may have a “cool” color temperature of about 3000K (appears pure white) at full brightness. Further, a standard bulb may have a “warm” color temperature of 2000K (appears amber/red) when dimmed to 5-10% of full brightness. The color temperature change of an incandescent light bulb generally follows the color change of a cooling black body (i.e., the Black Body Locus). People sometimes prefer this “warming” effect and dislike the non-color shifting dimming of LED lights.
In certain applications, it may be advantageous to independently control the color and the brightness. For example, it may be desired to have a relatively high brightness level for the LEDs, but have a perceived “warmer” color to the light emitted by the LEDs.
When the color temperature is adjusted, for example using a dimmer, the change in color can appear to the human eye to change or jump very quickly when a significant adjustment is made to the dimmer. This is undesirable. When adjusting the perceived color of the light emitted by the LEDs, it is preferred to “soften” or slow the rate of change in color temperature to allow the human eye to receive a gradual color change.
Finally, another problem with current systems is that when an LED light fixture in a group of fixtures fails this can have a significant negative effect on the control circuitry controlling the fixtures, even potentially damaging the other fixtures.
Therefore, what is desired is a lighting system suitable for LED lights which closely mimics the color curve and intensity curve of an incandescent light when dimming.
An object of the invention is to provide an LED lighting fixture that closely follows the dimming intensity of an incandescent light when dimming.
Another object of the invention is to provide an LED lighting fixture which mimics the warming color change of an incandescent bulb when the lighting fixture is dimmed.
Another object of the invention is to provide an LED lighting fixture with the above features and which provides a precise, “cool” light color that approaches a “white” light source when at full brightness.
Another object of the invention is to provide an LED lighting fixture having the above features and having the ability to dim in a smooth, gradual manner, without perceptible discrete steps or jumps in the level of light during dimming.
Another object of the invention is to provide an LED lighting fixture having the above features and having the ability to dim in a smooth, gradual manner, without perceptible, discrete steps or jumps in the perceived color of light during dimming.
Another object of the invention is to provide an LED lighting fixture having the above features which is operable with standard drivers for LED lighting fixtures.
Still another object of the invention is to provide an LED lighting system that provides for LED dimming along with perceived LED color shifting that mimics a standard incandescent lamp that is dimmable for substantially the entire range of a commercially available dimmer switch.
Yet another object of the invention is to provide an LED lighting system that provides for independent control of the intensity (brightness) of an LED light fixture and the perceived color of the light fixture, i.e., color temperature.
It is also desired to provide an LED lighting system that slows the rate of color change when the controller is adjusted to change the perceived color of light emitted by an LED light fixture.
It is also desired to provide an LED lighting system with the above features which can be controlled with standard controls, such as standard wall-mounted dimmer controls.
Finally, it is desired to provide an LED lighting system that protects the control circuit for adjusting the color and brightness of other LED light fixtures on the same dimmer when one or more individual fixtures fail.